Magnetic induction accelerator



March 10, 1953 R. WIDEROE 2,631,234

MAGNETIC INDUCTION ACCELERATOR Filed Dec. 22, 1947 Patented Mar. 10, 1953 UNITED PATENT OFFICE MAGNETIC. INDUCTION ACCELERATOR Rolf Wideriie; Zurich, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland-,1 ajoint-stock company A ipli'cationDecember 22, 1947", st ange. 7931062? In Germany November 20,1944

SectiomLflPublicLaw 69 August 8 1946l Patent: expires November 20, 1964 .p-roduces a magnetic fieldlvarying cyclically with time. and having. a. space distribution such that the injected electrons are. accelerated by the field round. and round. the tube on anorbitalfpat'h known as. the "equilibrium. circle. The. mag-- netic. fieldidividesinto two. components one com.- ponent called the inducing fielcl produces. the electron acceleration; the other component. called. the. control field produces acentripetal effect upon the electrons tooffset the centrifugal forces oi the electrons caused by their motion along. the circular. paththereby enabling the. electron stream. to be confined during. its entire period of acceleration to: anequilibrium. circle of substantially constant radius.- The electrons which. can be injected as the alternating current wavepasses through: zero accelerate to. an extremely high velocity by the time the ourrenthasreached its maximum value and can then be causedi'to-ime pinge upon a target to produce X-rays or'put tosome-other use:

In the accompanying. drawings which-serve to.

illustrate the invention; Fig..- 1- is a; view indiametricali section of induction. type electron accelerator provided with a conventional: type of electron injector; Fig.- 2 is ahorizontal sectional view of the electron injector: shown in Fig. 1 but drawn to an enlarged scale; Fig. 3- isa sectional view similar to'Fig. 2- illustrating the improved type of electron injector device embodyingv the principles of; theinvention; Fig. 311 is a sectional view similar to- Fig. 3 shewingi'a slight modification of: the Fig. 3: arrangement; Fig. 4 is aperspective: of the electron injector illustrated in Fig.v 3; Fig. 5' is a: sectional view of: a somewhat difierent embodiment: of the invention particu.-'- larly' adapted f-o'r'use withinduction. type acceler'ators where-successive streams of electrons are accelerated inopp'osite directionsalong the orbit on opposite halves. of the alternating; current. which produces the magnetic field; Fig. 6 is a 2: View in perspective of the electron injector' accordingto" Fig 5"; and Figs: 7-11 are sectibna-l views similar to Fig- 3" illustrating several other modified embodiments of the latterpos'sible' within'the scope of the invention;

A construction typical'of the'prior art i'sillus trated' in Figs. 1- a'nd'2f In Fig. l the induction accelerator viewed in diametra'l section isseen to be comprised of a magnetic field structure It made up from" steel Iamin'ations'o-f appropriate contour and. which includes" a pair of cylindrical p'ole's IIH" separated. byair gap I1 and Ibcateu' concentrically along; axis a-a, and a pair of other and' separated by" air" gap 1 4'. Yoke mem here 15" complete the magnetic circuit for the cyclically varyingflux set up in the annularand cylindricat poles. Poles" fl-l l" and 3--I3"' are surrounded by an annular winding preferably split into" two coil" sections l6*-l 6" connected in series for energi'zation from a" source of altern'ating current of suitable frequency as for example 10'0" cycles/sec. applied to terminals l'l'.

An annular. evacuated glass tube I 8 rests-in the air. gap [.4 between poles" l3-'t3" and thereby surrounds'the axial poles I l-l-l-. Located within the tube I8 is an'electron emi'ssiVe cathode 2 0 the axis of which is arranged parallel to the axis a-a'.' Cathode 2i)" is placed within a" semicylindrical shaped" focus'sing electrode (Wehencountered properly guiding; the electron stream. into the. equilibrium circle 231' from the cathode 20. Experience. has shown that this is mainly due to the design oflv the electrode assembly, particularly the accelerating.- electrode Z2 and: to the- Lorenz forces. impressed. upon' the electron stream by the control field that begins {:8 to exert its effect on the electrons as soon as they are emitted from the cathode.

Mathematically, this Lorenz force Kst is expressed by the equation:

where With the type of accelerating electrode 22 shown in Figs. 1 and 2, experience hasshown that the electron stream appears to originate at' an imaginary cathode 20a located outside of the accelerating electrode 22 and some distance behind the real cathode 20. The electron stream thus follows path 25 and after making one turn around the tube I8 it may very well happen that the entire stream or at least a part thereof under the action of the increasing magnetic control field will not have moved radially inward far enough to avoid a collision with the accelerating electrode 22 of the electron gun.

At best, only a small portion of the diverging stream of the electrons can be assured of reaching the equilibrium circle, the remainder being lost by dispersion.

, I have found that by redesigning the accelerating electrode, better control can be exercised over the initial turns made by the electron stream with the result that practically the entire stream will be certain to reach the equilibrium circle and there accelerated to its final velocity under the influence of the increasing induction field.

According to the invention, the single accelerating electrode 22 of Figs. 1 and 2 is replaced by separate electrodes (i. e. electrically independent) each being charged to a different potential. One of these electrodes is located to one side of the cathode between the latter and the equilib rium circle and is given a potential that is positive with respect to the cathode; the other elecr trode being located at the opposite side of the cathode is given a different positive potential for instance greater than that of the aforementioned first electrode with respect to the cathode.

Referring now to Figs. 3 and 4, the accelerating electrode structure is comprised of a pair of parallel rectangular plates 26, 21 located on each side of the Wehnelt cylinder 2| in which the cathode 20 is placed. Electrode plates 26, 21

lie in planes parallel with a plane containing the axis a-a of the induction accelerator and extend from a point rearwardly of the closed left end of electrode 2| to a point well in front of its open right end from which the electron stream emerges. As to the potentials impressed upon the elements of the electron gun, cathode 20 can for instance be placed at'l0 kilovolts,

'plate 26 nearest the equilibrium circle can be at O kilovolts, and plate 21 at +1 kilovolt. The

electrostatic field produced between the plates 26, 2'! by the different potentials impressed upon them will then be such as to cause the electron stream to be deflected radially outward i. e. in the direction of arrow 28. borhood of the cathode 20, this field is distorted In the direct neighand on the electron emissive surface 20 itself the field is equal to zero. The force produced by the electric field between plates 26, 21 thus increases from zero at the cathode 20 in the direction of stream emission until a maximum value is reached. This radially outward directed force accordingly follows a law similar to that followed by the radially inward directed Lorenz force of the guide field which likewise begins to increase at the instant 'of electron emission, the velocity v in Equation 1 of the emitted electrons being very small at the surface of the cathode, and is therefore effective to prevent the electron stream from taking an initial path of the wrong curvature as already explained. The two forces generally compensate one another with the result that the electron stream leaves the space between plates 26, 2'! on an inward spiral path such as indicated by the curve 29 that will assure a clear and uninterrupted run into the equilibrium circle after a few turns around the tube l8.

Instead of the plane flat plates 26, 21, a pair of juxtaposed curved electrodes 26', 21' may be employed as shown in Fig. 3a.

If the induction accelerator be of the type such as shown in my co-pending United States application, Ser. No. 720,544 filed January '7,

1947, now U. S. Patent No. 2,538,718 issued January 16, 1951, wherein two electron streams are accelerated in opposite directions around the tube on successive half waves of the applied alternating current, the present invention can be applied to each stream merely by replacing the semi-closed Wehnelt cylinder 2| of Figs. 3 and 4 with a two-partelectrode 30 open at both ends as shown in Figs. 5 and 6, the accelerating elec' trode plates 26, 21 being as before.

The compensating electrostatic field produced between plates 26 and 21 may be varied either by changing the radial distances 0 and d between the plates and the cathode or by chang ing the contour of the plates as shown in Figs. '7 to 11 or by using both of these expedients to suit particular operating conditions encountered.

In the construction shown in horizontal section in Fig. 7, the inner and outer plane electrode plates 3|, 32 are each provided with an outwardly bent portion 3| a, 32a that establishes a divergent passage therebetween for the elec tron stream.

In the constructional embodiments illustrated in horizontal section in Figs. 8-11, the electrode located at the side'of the cathode nearestthe equilibrium circle is a rectangular plane plate 34 while the other plate is curved or bent to alter the strength of the electrostatic field at successive points along the path taken by the stream as it passes between the plates. In Fig. 8, the outer plate 35 has an outwardly bent plane wall section 35a beginning at the radius passing through the cathode 28 and the axis a-a. The Fig. 9 construction is similar to that shown in Fig. 8 except that the bent portion 36a of the plate 36 s is offset radially inward and begins at a point immediately in front of the mouth of electrode 2 In Figs. 10 and 11, the outer electrode plate has a portion which arches inwardly towards the stream beginning at a point located substantially at the mouth of electrode 2| and then outwardly away from the stream. In Fig. 10, the curved portion 31a of the outer plate 31 conf-ormsto the arc of a relatively large circle while in Fig. 11, the

curved plate portion 38a of electrode 38 follows the arc of a circle of much smaller radius.

The two halves of the accelerating electrode such as the plates 26, 21 in Figs. 3 and 4 can also be used for imparting to the electron stream an initial radial speed component so that in the event the position of the cathode in a radial direction is not exactly correct with respect to the induction and control field structure, the stream will still be guided into the equilibrium circle along a satisfactory inwardly spiraling path. In order to attain this, it may even be necessary to set the outer plate 21 at a potential somewhat less positive than that applied to the inner plate 26. Preferably the potential applied to at least plate 2! is made adjustable such as by rheostat 39 so that it can be set to the value best suited for a particular operating condition of the accelerator.

In conclusion, while I have described and illustrated certain preferred embodiments of the invention, it is to be understood that the exact constructions shown in the drawings are exemplary and that accordingly these may be changed by others when practising the invention without departing from the spirit and scope of th latter as defined in the appended claims.

I claim:

1. The combination with a magnetic induction accelerator for charged particles such as electrons including an annular chamber in which the electrons are accelerated to high velocity along a circle of equilibrium under the combined action of a magnetic inducing field and a magnetic control field whose respective field strengths vary with time and an electron emissive cathode located within said chamber to one side of said circle and from which electron streams are periodically emitted in timed relation with the variation in said fields, of electrode means for imparting an initial velocity to the electron stream, said electrode means being comprised of a pair of electrically independent electrode members located on each side of said cathode and which when charged establish therebetween an electrostatic field through which th electron stream passes en route to the equilibrium circle, and a charging input line individual to each of said electrode members, said electrode member nearest said circle being adapted to be charged through its associated input line to a potential positive in relation to that of the cathode and said other electrode member being adapted to be charged through its associated input line .to a different positive potential in relation to that of the cathode.

2. A magnetic induction accelerator as defined in claim 1 wherein the cathode is located within a Wehnelt cylinder having a potential substantially the same as that of the cathode.

3. A magnetic induction accelerator as defined in claim 1 wherein the cathode is located within a semi-cylindrical shaped electrode having a potential substantially the same as that of the cathode.

4. A magnetic induction accelerator as defined in claim 1 and means for adjusting the potential of at least the electrode member farthest from the equilibrium circle.

5. A magnetic induction accelerator as defined in claim 1 wherein the spacing between said electrode members varies in a direction substantially transverse to the emission path of the electron stream.

6. An accelerator for electrons comprising the combination of an annular evacuated tube providing a path for the acceleration of electrons, means for providing a time-varying magnetic field for causing acceleration of electrons in said tube, an electron source, means for projecting electrons from said source into said field and an electrode located adjacent the emergence of said electrons for guiding said electrons into a direction substantially parallel to the orbital path of said electrons in said tube.

ROLF WIDERE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,331,788 Baldwin Oct. 12, 1943 2,394,073 Westendorp Feb. 5, 1946 2,484,549 Blewett Oct. 11, 1949 

